This invention relates to oscillator frequency stabilization circuits, and it relates more particularly to electric circuits of that type which are able to operate without the need for phase information in received signals with respect to which they are stabilized.
One factor commonly discouraging the usage of single sideband techniques in cellular mobile radiotelephone systems has been the difficulty of economically stabilizing oscillators at mobile radio stations to a predetermined frequency with sufficient precision to assure signal quality similar to that generally experienced in fixed station systems. The difficulty arises at least in part from the need to compensate for local oscillator drift as compared to a signal having a rapid frequency modulation, e.g., modulation due to Doppler frequency shift effects in a multipath radio signal transmission medium. Furthermore, the drift in frequency is partly due to component aging and partly due to more rapidly changing temperature conditions such as may occur when a vehicle with a mobile radio unit is initially started up on a cold day.
In land mobile radiotelephone systems, there are strong multipath effects. That is, at a given antenna, there are received simultaneously plural versions of a single message transmission propagated by way of various paths including different delays because of reflections from different topological features of the respective paths. Thus, the amount of net frequency shift due to Dopper effects at any instant of reception is unpredictable and varies rapidly in a frequency range which is as large as, for example, .+-.80 Hz for a mobile unit moving at 60 miles per hour and having its radio working in a frequency range above 800 mHz. Because of this unpredictability, the frequency modulation is sometimes called random frequency modulation or Doppler frequency spread, i.e., Doppler shift in a changing multipath environment. The term "Doppler spread" is generally utilized herein.
Fast acting phase locked loops track such fast random frequency variations instead of stripping them away. The Doppler spread must be removed to provide a stable time base for radio receiver operation. Other loops that operate more slowly have various faults such as not being conveniently useful with digital processing systems or providing an error signal that is inversely proportional to the frequency level at which the system works.
Frequency stabilization systems in the prior art generally are not concerned with land mobile radio communication problems and so do not deal with the fast-swinging frequency shifts which characterize the Doppler spread phenomenon. To the extent that the Doppler frequency shift effect is sometimes considered in signal receivers, it is in an environment which involves relatively uncomplicated and predictable shift effects. One example of prior art which considers Doppler shift without reference to the multipath problem is an A. L. Lindstrum, U.S. Pat. No. 3,983,501, wherein phase shift keyed signals are being transmitted in an undersea environment; and an adjustable bandwidth filter is included in a phase locked loop. Another reference is the T. F. Haggai, U.S. Pat. No. 3,346,814, in which a phase locked loop demodulator is included in a frequency stabilization path prior to the integrator of an automatic frequency control loop which provides an output for correcting a voltage controlled oscillator for low frequency drifts or variations. The corrections maintain the average carrier frequency centered in the passband of an input selectable-bandwidth filter bank but the demodulator loop oscillator can drift out of the passband of the filter.
Several frequency tracking loops use various digital counting algorithms which do not conveniently provide high stability in the environment of both fast and slow frequency changes. For example, the J. R. Zoerner, U.S. Pat. No. 3,370,252 and an L. Grohmann, U.S. Pat. No. 3,922,609, use a counter to generate a pulse after counting a certain number of received signal cycles and then they employ analog circuits to operate on the pulses to develop an analog error signal. In an A. B. Lyberg, U.S. Pat. No. 4,048,581, a digital error signal appears in a reversible counter, the output of which is utilized to control local oscillator frequency. However, that error signal resulted from using the counter to digitalize an analog error signal generated by other means not including the counter.